Method and system for forming birefringent voxels

Abstract

A method for forming a birefringent voxel involves simultaneously generating a first seed pulse and a first data pulse. The first seed pulse and the first data pulse are spatially separated laser pulses having different amplitudes. The first seed pulse is focused to a first seed location, and the data pulse is focused to a first data location. The first seed location and the first data location are separated by a predetermined distance along the scanning path, with the first seed location being in front of the first data location. Subsequently, a second seed pulse and a second data pulse are generated and focused to a second seed location and a second data location, respectively. The second seed location and the second data location are separated by a predetermined distance. The second data location is the same as the first seed location, thereby resulting in the formation of a birefringent voxel.

Description

[Background technology]

[0001] background

[0001] There is considerable demand for data storage. It is estimated that in the near future, cloud storage providers will need data storage capacity on the order of zettabytes (a zettabyte is 1 trillion gigabytes (10)). 21 It is a byte. The majority of the data needs to be stored for long periods of time.

[0002]

[0002] Examples of currently widely used data storage technologies include hard disk drives, magnetic tape, flash memory, and optical discs. All of these technologies have the drawback that data must be periodically copied to a replacement medium. This is costly in terms of both energy consumption and hardware requirements.

[0003]

[0003] Magnetic storage media such as hard drives and magnetic tapes have the problem of gradually demagnetizing. Flash memory is susceptible to the read disturb effect, in which repeated reading from a specific flash cell causes damage to surrounding flash cells. Reflective materials used for data storage in optical media such as DVDs deteriorate over time.

[0004]

[0004] As a solution to these drawbacks, birefringent optical data storage media have been proposed. Birefringent optical data storage media comprises a transparent substrate such as a quartz glass substrate. Data is encoded in three-dimensional nanostructures formed within the substrate. These nanostructures are called voxels.

[0005]

[0005] The voxels have optical properties different from those of the surrounding bulk substrate. In particular, the voxels are birefringent, i.e., exhibit different refractive indices depending on the polarization and / or direction of the incident light. The optical properties of the voxels can be controlled when the voxels are written into the substrate. The optical properties of the voxels are used to encode data.

[0006]

[0006] Birefringent optical data storage media and their manufacture are described, for example, in Anderson et al, Glass: A New Media for a New Era? 10th USENIX Workshop on Hot Topics in Storage and File Systems (HotStorage18), 2018 and U.S. Patent No. 10,236,027B1.

Summary of the Invention

Means for Solving the Problems

[0007] Summary

[0007] In one aspect, a method of forming birefringent voxels within a transparent substrate is provided. The method includes simultaneously generating a first seed pulse and a first data pulse, wherein the first seed pulse and the first data pulse are spatially separated laser pulses and the amplitude of the first seed pulse is different from the amplitude of the first data pulse; focusing the first seed pulse at a first seed location within the substrate and focusing the data pulse at a first data location within the substrate, wherein the first seed location and the first data location are separated by a predetermined distance along a scanning path and the first seed location is in front of the first data location in the scanning path; After focusing the first seed pulse at the first seed location and the data pulse at the first data location, generating a second seed pulse and a second data pulse simultaneously, wherein the second seed pulse and the second data pulse are spatially separated laser pulses and the amplitude of the second seed pulse is different from the amplitude of the second data pulse; Focusing the second seed pulse at a second seed location within the substrate and the data pulse at a second data location within the substrate, wherein the second seed location is separated from the second data location by a predetermined distance along the scan path, the second seed location is in front of the first seed location and the second data location in the scan path, and the second data location is the same as the first seed location, thereby resulting in the formation of a birefringent voxel; including.

[0008]

[0008] Related aspects provide a transparent optical data storage medium obtained by the present method.

[0009]

[0009] Another aspect provides a system for forming birefringent voxels within a transparent substrate. The system includes a pulsed laser source, a beam splitter disposed downstream of the pulsed laser source along the optical path, and a focusing optical system located downstream of the beam splitter on the optical path. The beam splitter includes a phase retarder, a first polarization-selective diffractive optical system located downstream of the quarter-wave plate along the optical path, and a second polarization-selective diffractive optical system located downstream of the first polarization-selective diffractive optical system along the optical path. The system is useful for implementing a method as described herein.

[0010]

[0010] Yet another aspect provides a beam splitter. The beam splitter includes a phase retarder, a first polarization-selective diffractive optical system located downstream of the phase retarder along the optical path, and a second polarization-selective diffractive optical system located downstream of the first polarization-selective diffractive optical system along the optical path. The beam splitter may enable the generation of split laser pulses having a controllable amplitude ratio.

[0011]

[0011] This summary is provided to introduce some of the concepts that will be further described below in the detailed description in a simplified form. This summary is not intended to identify the main or essential features of the claimed invention, nor is it intended to be used to limit the scope of the claimed invention. Furthermore, the claimed invention is not limited to any implementation that solves any or all of the disadvantages described herein.

[0012] Brief explanation of the drawing

[0012] To aid in understanding embodiments of the present disclosure and to illustrate how such embodiments may be carried out, attached drawings are referenced merely as examples. [Brief explanation of the drawing]

[0013] [Figure 1]

[0012] This is a schematic cross-sectional view of a birefringent optical data storage medium. [Figure 2]

[0012] This is a flowchart illustrating an overview of a method for forming birefringent voxels in a transparent substrate. [Figure 3A]

[0012] Figure 2 shows optical data storage media after various iterations of the method. [Figure 3B]

[0012] Figure 2 shows optical data storage media after various iterations of the method. [Figure 3C]

[0012] Figure 2 shows optical data storage media after various iterations of the method. [Figure 3D]

[0012] Figure 2 shows optical data storage media after various iterations of the method. [Figure 4]

[0012] This is a diagram of a raster scanning pattern used in a method for forming birefringent voxels. [Figure 5]

[0012] This is a schematic plan view of the optical data storage medium obtained by following the scanning path in Figure 4. [Figure 6]

[0012] This is a block diagram of a write head that is useful for implementing the method shown in Figure 2. [Figure 7]

[0012] Annotated schematic diagram of a polarizing grating-based beam splitter. [Figure 8A]

[0012] This is a scanning electron microscope (SEM) image of a voxel obtained by the comparison process. [Figure 8B]

[0012] This is an SEM image of a voxel obtained by the method shown in Figure 2. [Figure 9]

[0012] This is a plot of data capacity versus the number of laser pulses per voxel. [Figure 10A]

[0012] This is a plot of the data capacity as a function of the walk-off distance in the comparison method. [Figure 10B]

[0012] This is a plot of the data capacity as a function of the walk-off distance according to the method shown in Figure 2. [Figure 11]

[0012] Figure 7 shows a plot of the beam separation angle as a function of the angle between the two polarization gratings of the beam splitter. [Figure 12]

[0012] This is a plot of the intensity ratio of the two beams output by the beam splitter shown in Figure 7 as a function of the angle of the quarter-wave plate. [Modes for carrying out the invention]

[0014] Detailed explanation

[0013] First, the structure of an exemplary birefringent optical data storage medium will be described with reference to Figure 1, which is a schematic cross-sectional view of an optical data storage medium.

[0015]

[0014] The data storage medium 100 comprises a substrate 110 in which birefringent voxels 120 are embedded.

[0016]

[0015] The substrate 110 may include glass, particularly fused silica. Fused silica is sometimes also called silica glass. Glass has excellent chemical and thermal stability, and data storage media based on glass substrates have an expected lifespan of several hundred years. Therefore, optical data storage media are useful for long-term storage of data.

[0017]

[0016] The shape and dimensions of the substrate are not particularly limited. Figure 1 shows a rectangular parallelepiped substrate. In modified forms, the substrate may have any shape (for example, cylindrical).

[0018]

[0017] The substrate generally has a thickness t of up to 10 mm in the z direction, optionally up to 5 mm, and optionally in the range of 200 μm to 2 mm. More generally, the thickness of the substrate may be appropriately selected based on the capabilities of the imaging system used to image the voxels. The z direction is the direction in which light passes through the substrate when imaging the voxels. The attenuation of light passing through the substrate increases as a function of the distance traveled, and such attenuation can be limited by providing a relatively thin substrate.

[0019]

[0018] Multiple voxels 120 are embedded in the substrate 110. A voxel is a discrete volume of the substrate that has been modified to have non-inherent birefringence, that is, birefringence different from that of the bulk substrate material.

[0020]

[0019] A voxel has a position that can be described by a set of coordinates x, y, z. A voxel may exhibit birefringence, that is, it may have different refractive indices for light of different polarizations. A voxel can cause changes in the polarization angle and ellipticity of light. A voxel has linear retardance. Linear retardance is a measure of the magnitude of the phase shift between specific orthogonal linear polarization components of light after it has passed through a voxel. The direction of the electric field oscillation of a particular orthogonal polarization component that gives a larger phase shift is called the azimuthal angle of birefringence.

[0021]

[0020] When writing voxels to a substrate, the position, retardation, and polarization angle of the voxels can be controlled. Digital data can be encoded using any one of these properties, or any combination thereof.

[0022]

[0021] In the illustrated example, the voxels 120 are arranged as a stack of layers. The stack comprises an upper layer 130, an intermediate layer 132, and a lower layer 134. Any number of voxel layers may exist. For example, an optical data storage medium may contain 1 to 100 voxel layers.

[0023]

[0022] Voxels within a single layer are separated from each other laterally. The distance between adjacent voxels within a single layer is sometimes called the pitch p. Generally, the smaller the spacing between voxels, the higher the data density that can be stored per unit area in the optical data storage medium.

[0024]

[0023] In order to form birefringent voxels within the substrate, at least two laser pulses are focused at a location within the substrate. Each laser pulse causes photoexcitation of the material, inducing a physical change in the substrate material at that location.

[0025]

[0024] The substrate is most commonly a transparent substrate. In this mounting configuration, it is possible to form voxels on the surface of the substrate, but voxels are usually embedded within the main body of the substrate.

[0026]

[0025] The use of opaque or translucent substrates is also intended. In such mounting configurations, the voxels are formed on the surface of the substrate.

[0027]

[0026] The voxels formed on the surface are sometimes called laser-induced periodic surface structures ("LIPSS").

[0028]

[0027] The first pulse is called the "seed pulse." The seed pulse causes a seed modification, which may be a nanovoid or any other isotropic structure.

[0029]

[0028] The second pulse and any subsequent pulses are called "data pulses." One or more data pulses are one or more polarized laser pulses focused at the location of the seed modification. One or more data pulses modify the structure of the seed modification, thereby generating a birefringent voxel.

[0030]

[0029] One or more data pulses may have a linearly polarized state, an elliptically polarized state, or a circularly polarized state. The birefringence of the voxel is determined by the polarization of one or more data pulses. For example, the azimuthal angle of birefringence can be controlled by changing the polarization angle of one or more data pulses. This enables polarization multiplexed optical data recording.

[0031]

[0030] The seed pulse has a higher amplitude than the data pulse (i.e., it is a higher energy laser pulse).

[0032]

[0031] A delay is required between the seed pulse and the data pulse in order to avoid thermal damage to the substrate.

[0033]

[0032] There are two existing methods for forming birefringent voxels, called "hot writing" and "cold writing," respectively.

[0034]

[0033] In the hot writing method, seed pulses and data pulses used to form a particular voxel are applied sequentially through the same optical path. These steps are then repeated for each voxel to be written. To improve throughput, the substrate is moved during the writing process.

[0035]

[0034] In order for a voxel to be formed properly, the seed pulse and data pulse must be focused to points that are sufficiently close to each other. The distance between the focus of the seed pulse of a voxel and the focus of the last data pulse of that voxel is called the "walk-off distance".

[0036]

[0035] The acceptable “walk-off distance” limits the speed at which the focus of the writing head can move and requires precise relative positioning of the substrate and the writing head. In the hot writing technique, the “walk-off distance” is given by dividing the speed at which the focus moves relative to the substrate by the pulse rate of the laser light source. The acceptable walk-off distance is often less than one-tenth of the voxel pitch.

[0037]

[0036] Typical laser light sources operate at a uniform pulse rate on the order of tens of megahertz. The maximum focal point movement speed relative to the substrate is slower than the laser pulse rate. As a result, many laser pulses must be wasted. In typical hot lighting implementations, approximately 80% of laser pulses are wasted.

[0038]

[0037] Furthermore, since the seed pulse and data pulse have different amplitudes, the hot lighting process requires high-speed modulation of the laser pulse amplitude. High-speed modulation introduces considerable signal noise and instability.

[0039]

[0038] In the cold lighting method, the medium is prepared by forming a seed modification in the substrate during the first scan. Then, a data pulse is applied to the seed modification during the second scan.

[0040]

[0039] Cold lighting techniques overcome some of the limitations of hot lighting. High-speed modulation is not required, thereby reducing hardware costs and eliminating one of the signal noise sources.

[0041]

[0040] Nevertheless, cold lighting still has some limitations. Because it requires two scans, the maximum possible throughput of the cold lighting method is half the laser pulse rate. Aligning the data pulse with the seed modification is difficult, and becomes increasingly difficult as the scanning speed increases. Furthermore, there is a considerable delay between the formation of the seed modification and the application of the data pulse, which makes the technique susceptible to temporal variations in the calibration of the writing system.

[0042]

[0041] This specification provides a method for forming birefringent voxels in a transparent substrate, which can address one or more of the limitations of existing methods.

[0043]

[0042] This method is called "pseudo-single pulse writing" and involves splitting a single laser pulse into two or more spatially separated pulses having independently modulated amplitudes. These pulses are delivered to the transparent substrate simultaneously. At least one of the split pulses acts as a seed pulse, and at least one of the split pulses acts as a data pulse. These steps are repeated while scanning along the scanning path in the transparent substrate. The data pulses of later iterations reach the location of the seed modification formed by the seed pulse of the previous iteration, thereby forming a birefringent voxel.

[0044]

[0043] This method allows voxels to be written to the substrate at a rate approximating the laser pulse rate, thereby improving throughput. This method reduces the number of wasted laser pulses, thereby improving hardware utilization efficiency and reducing energy consumption.

[0045]

[0044] The method will now be explained with reference to Figures 2 and 3A to 3D. Figure 2 is a flowchart showing an overview of the method. Figures 3A to 3D show the state obtained after a series of iterations of the method.

[0046]

[0045] This method is an iterative method. The number of iterations to be performed is N. s N is a non-zero integer corresponding to the number of seed modifications formed within the substrate. As will become clear from the following explanation, s This value is equal to the number of birefringent voxels written plus a non-zero integer constant.

[0047]

[0046] Each iteration of this method includes the operation of blocks 201 and 202.

[0048]

[0047] In block 201, a seed pulse S and a data pulse D are generated simultaneously. The operation of this block generally involves generating a source laser pulse using a laser source and then splitting the source laser pulse into a seed pulse and a data pulse. The use of two or more laser sources to generate a synchronization pulse is also intended.

[0049]

[0048] The laser source may be a femtosecond laser. The laser source has a time interval t p It may also be a pulsed laser source that generates laser pulses.

[0050]

[0049] The properties of the beam splitter are not particularly limited and can be selected as appropriate. The laser pulse may be split by passive beam splitting techniques or active beam splitting techniques. Exemplary techniques include the use of a polarizing grating, a polarizing beam splitter, an acousto-optic deflector, or a spatial light modulator.

[0051]

[0050] The seed pulse and the data pulse each have different amplitudes.

[0052]

[0051] The seed pulse has an amplitude selected to form a seed modification within the transparent substrate. The seed modification can be any modification to the structure of the transparent substrate that results in a local change in the refractive index of the substrate compared to the refractive index of the initial unmodified substrate. The seed modification is generally isotropic; that is, it has the same refractive index regardless of the polarization of the light used to probe the seed modification.

[0053]

[0052] Examples for describing seed modification include the generation of high-density or low-density areas, localized changes in the glass matrix, crystallization or amorphous formation, and the formation of nanoscale voids.

[0054]

[0053] The seed pulse may be polarized or unpolarized.

[0055]

[0054] The data pulse has an amplitude selected to modify the seed modification in order to induce birefringence. The amplitude of the data pulse is generally lower than the amplitude of the seed pulse.

[0056]

[0055] The data pulse is polarized.

[0057]

[0056] The method may further include modulating the polarization of the data pulse. For example, the data pulse may pass through a polarization state generator. By modulating the polarization of the data pulse, the birefringence characteristics of the voxel formed by the data pulse can be changed.

[0058]

[0057] The polarization of the data pulse may be set individually for each iteration of this method.

[0059]

[0058] In principle, two or more data pulses may be used, but using two or more data pulses per voxel does not offer any particular advantage. The inventors have found that the azimuth angle of a birefringent voxel is determined by the last data pulse that reached that voxel.

[0060]

[0059] When the seed pulse and data pulse reach the transparent substrate, they are spatially separated by a predetermined distance.

[0061]

[0060] The predetermined distance may have a fixed value for all iterations of steps 201 and 202. The predetermined distance is jp v It may also be the case that j is the interval coefficient and pv is the scanning direction V of the optical data storage medium. f This is the voxel pitch in the . The spacing coefficient j is a non-zero integer, and may be in the range of, for example, 1 to 5.

[0062]

[0061] In order to avoid or reduce crosstalk between the seed pulse and the data pulse, the given distance is generally at least twice the wavelength of the laser light.

[0063]

[0062] In block 202, the seed pulse and data pulse are focused to their respective locations within the transparent substrate. The seed pulse and data pulse are focused to a predetermined distance jp v Only the seed pulse is separated, and the scanning direction V f It is located before the data pulse.

[0064]

[0063] The seed pulse and data pulse may pass through the same optical component before reaching the transparent substrate. This allows for more consistent relative positioning of the seed pulse and data pulse, as any variation in the operating parameters of the component equally affects both the seed pulse and the data pulse.

[0065] Instead, the seed pulse and the data pulse can follow different optical paths. For example, the data pulse may pass through a polarization state generator, and the seed pulse may bypass the polarization state generator. Thereby, a small difference (e.g., 1 m or less, more generally 10 cm or less) can be introduced between the path length of the seed pulse and the path length of the data pulse. The difference in arrival times is small enough to be negligible compared to the speed of movement of the substrate relative to the write head, so pulses passing along different paths are still considered to arrive simultaneously.

[0066]

[0065] By focusing the seed pulse at a location within the transparent substrate, a seed modification is formed at that location.

[0067]

[0066] By focusing the data pulse at the location of the seed modification within a small tolerance range called the walk-off distance, a birefringent voxel is formed. As an illustration, a typical maximum walk-off distance can be up to 100 nm.

[0068]

[0067] By focusing the data pulse at a location other than the location of the seed modification, a weak modification may be formed or no modification may be formed.

[0069]

[0068] In the flowchart of FIG. 2, in decision block 203, it is tested whether all desired birefringent voxels have been written to the transparent substrate. If so, the method proceeds to block 204 and ends.

[0070]

[0069] If not, the method performs further iterations of the steps of blocks 201 and 202. In a further iteration, as shown in block 205, the foci of the seed pulse and the data pulse are moved by a distance p corresponding to the voxel pitch in the scanning direction V f in the scanning direction. v by a distance corresponding to the voxel pitch in the scanning direction.

[0071]

[0070] FIGS. 3A - 3D show exemplary states reached after a series of iterations of the above method.

[0072]

[0071] The value of index i is used to track the iteration being described. It follows the rule of zero-indexing. The value of i is incremented by 1 in each iteration. The use of an index may be useful in implementations in which this method is implemented as a computer-controlled process.

[0073]

[0072] Figure 3A shows the zeroth iteration (i=0) of the method in Figure 2.

[0074]

[0073] At time t=0, the write head 310 (which may be a write head 600 as described later with reference to Figure 6) focuses the data pulse D0 and the seed pulse S0 to their respective locations on the substrate 320. As described above, the data pulse and the seed pulse are generated and delivered to the substrate simultaneously.

[0075]

[0074] The data pulse D0 reaches the unmodified portion of the substrate and forms the zeroth weak modification 330(0) within the substrate 320.

[0076]

[0075] The seed pulse S0 is in the scanning direction V f distance jp v A weaker modification 330(0) is formed in the position before the seed modification 340(0) (p v (Remember that j is the voxel pitch in the scanning direction of the completed optical data storage medium, and j is a non-zero integer). In the examples shown in Figures 3A-3D, j is 2.

[0077]

[0076] There are still voxels to be written. In this method, the writing head is aimed in the scanning direction V f At the distance p v The process proceeds via block 205, which is shifted by a certain amount of time. Moving the target of the write head may include adjusting the scanning optical system of the write head 310 and / or moving the substrate and / or the write head relative to each other.

[0078]

[0077] The method proceeds to the next (first, i=1) iteration shown in Figure 3B.

[0079]

[0078] The first iteration is at time t p It occurs in t. p This is the time interval between pulse generation by the laser light source of the write head 310. In other words, the first iteration uses the source laser pulse generated immediately after the source laser pulse used in the zeroth iteration.

[0080]

[0079] According to blocks 201 and 202 in Figure 2, data pulses D1 and seed pulses S1 are generated and focused to their respective positions within the substrate 320.

[0081]

[0080] The data pulse D1 is directed towards the unmodified region of the substrate, forming a further weak modification 330(1). The further weak modification 330(1) is in the scanning direction V f In the early weak modification 330(0), the pitch p v It is located a few minutes downstream.

[0082]

[0081] The seed pulse S1 is in the scanning direction V f In this case, the initial seed modification is 340(0) and the pitch is p v This forms a further seed modification 340(1) located a few minutes downstream.

[0083]

[0082] No voxels are formed after this iteration. The writing head is aimed in the scanning direction V f At pitch p v The process is incremented by min, and the method proceeds to the second (i=2) iteration shown in Figure 3C.

[0084]

[0083] The second (i=2) iteration is time point 2t p It is generated at [location] and uses a laser pulse generated immediately after the laser pulse used in the first (i=1) iteration.

[0085]

[0084] In this case as well, the source laser pulse is located further in the scanning direction than the target position of the preceding D1 pulse and S1 pulse. v It is divided into a data pulse D2 and a seed pulse S2 directed at a certain distance.

[0086]

[0085] The seed pulse S2 is directed towards the blank area of ​​the substrate to form the seed modification 340(2).

[0087]

[0086] The index of this iteration is equal to a coefficient j representing the interval between the data pulse and the seed pulse. The data pulse D2 reaches the location of the seed modification S0 in the zeroth (i=0) iteration of the method. Rather than forming a weak modification 330, the data pulse D2 modifies the seed modification 340(0) to form a birefringent voxel 350(0). By modulating the polarization of the data pulse D2, the birefringence of the voxel 350(0) can be adjusted as desired to encode the data symbol.

[0088]

[0087] As can be understood, the data pulse used to form voxel 350(0) reaches the seed modification some time after the formation of the seed modification. This allows the substrate time interval jt p This is provided, thereby avoiding thermal damage to the substrate. For example, with a laser repetition rate of 50 MHz, the period t between laser pulses is given. p The interval is 20 ns. The interval factor j of 5 provides a cooling time of 100 nanoseconds between the arrival of the seed pulse and the arrival of the data pulse at any given location.

[0089]

[0088] The cooling time can be appropriately selected based on the selected material, with interval coefficient j and / or laser pulse interval t. p This can be controlled by changing the following parameters. By changing the interval coefficient j among these parameters, it may be possible to adjust the cooling time without sacrificing data throughput.

[0090]

[0089] This method continues further iterations as long as there are voxels to be written to.

[0091]

[0090] A generalized iteration i of this method uses the i-th pulse from the laser source to time it p It occurs in the following location. The pulse from the laser source is divided into a data pulse and a seed pulse. The data pulse is transmitted through the scanning path V f IP address according to v It is delivered to the unit position. The seed pulse is (i+j)p along the scanning path. v A seed modification is formed at the unit position. If the iteration count i is greater than or equal to the interval coefficient j, the scanning direction V f Position along (ij)p v Birefringent voxels are formed in this region; otherwise, the data pulse reaches the blank region and forms a weak modification.

[0092]

[0091] The product of the final iteration is shown in Figure 3D.

[0093]

[0092] This method forms an optical data storage medium 320 having j weak modifications within the data margin 360. The scanning direction V used when writing to the substrate f Downstream of the data margin in (N s There exists a data array 370 containing -2j voxels.

[0094]

[0093] The birefringence of each voxel in the data array can be controlled independently by modulating the polarization of the corresponding data pulse, thereby resulting in voxels that encode different data symbols, as shown by the different shading patterns in Figure 3D.

[0095]

[0094] Downstream of the data array 370, there is a seed margin 380 that includes j non-birefringent seed modifications.

[0096]

[0095] When the method is operating under steady-state conditions, that is, after the data margin has been formed and before the seed margin has begun to form, the method can make good use of all pulses generated by the laser source to form voxels. In particular, the overhead is small because j is small compared to the number of voxels with no upper limit (for example, in the range of 1 to 5).

[0097]

[0096] Various modifications may be made to the illustrated example.

[0098]

[0097] In this example, j is a non-zero integer. This minimizes the walk-off distance, thereby improving the quality of the voxels and, consequently, increasing the number of bits that can be stored per voxel. In a variant, j may be a non-integer value, provided that the walk-off distance is not so large as to hinder the formation of voxels.

[0099]

[0098] This example has described how to write one row of voxels. Two or more rows may be written. If two or more rows are written, they may be written in parallel and simultaneously.

[0100]

[0099] This example shows the relative movement of the write head and the substrate. More generally, any technique or combination of techniques that allows adjustment of the aiming of the write head may be used. Aiming may be adjusted by adjusting the orientation of the beam scanner of the write head, moving the write head, moving the substrate, and / or adjusting the depth of focus of the write head.

[0101]

[0100] This example shows the writing of a single voxel layer. Multiple voxel layers may be written to increase the data storage capacity per unit volume.

[0102]

[0101] In principle, the seed margin can be omitted by directing data pulses to additional seed modifications within the seed margin. However, this may involve modifying the write head, which may not be cost-effective.

[0103]

[0102] In this example, the scan path is linear. In modified configurations, scanning can follow any suitable path. The scan path may be helical, particularly in mounting configurations where the substrate is cylindrical or disc-shaped. The scan path may be a raster scan path. The scan path may be a meandering scan path, i.e., a scan path with a square wave shape. When following a scan path that involves reversing the scanning direction, one or more optical components of the write head (e.g., a beam splitter) are adjusted so that the seed pulse remains positioned before the data pulse.

[0104]

[0103] This example shows pulses separated in a direction perpendicular to the propagation direction of the laser light. In modified forms, spatial separation may be alternatively or additionally in the propagation direction. Polarizing lenses may be used to achieve separation in the propagation direction.

[0105]

[0104] The described method may be parallelized, and several instances of this method may occur simultaneously. Parallelization may involve the use of an additional beam splitter to divide the source laser pulse into two or more write paths, each write path configured to generate a seed pulse and a data pulse from the input pulse. Alternatively, each parallel write path may have its own laser source.

[0106]

[0105] Figure 4 shows an example of raster scanning.

[0107]

[0106] In the illustrated example, the substrate 420 is at speed V s The device is continuously translated in the -x direction. The write head moves along the y-axis. Seed pulses and data pulses propagate in the z direction.

[0108]

[0107] The write head writes a row of voxels using the method described with reference to Figures 2 and 3, by first linear scanning path V in the -y direction f The scan is performed along the path. The write head then moves in the +y direction along the reset path R relative to the substrate 420 and returns to the home position. No voxels are written when the write head returns to the home position. This scan pattern continues until all desired voxel lines have been written. s It repeats in this cycle.

[0109]

[0108] This scanning process creates lines of birefringent voxels (sometimes called scan lines). Adjacent scan lines are separated by a distance V s T s They are separated by a distance of p. The voxels within the scan line are separated by a distance of pitch p, and this pitch p v This is the period t of the laser source. p And, row V f It is the product of the scanning speeds. The separation between scan lines is V. s T s The pitch p between voxels in the scan line is the pitch between voxels. v It can be different.

[0110]

[0109] As a result of the continuous movement of the substrate and the writing head, the scan lines are V with respect to the y axis. s / V f It slopes by 1 minute. s / V f These are generally on the order of a few milliradians. Scanning direction V s This incline can be reduced or eliminated by angling the substrate relative to it.

[0111]

[0110] Figure 5 shows the optical data storage medium 520 obtained by the scanning process in Figure 4. In the ideal case, the voxels are arranged on equally spaced scan lines, and each scan line is consistently angled with respect to the substrate.

[0112]

[0111] As shown in the figure, the geometry of the scan lines of voxel 550 can deviate from the ideal case. Factors such as tolerances in mechanical alignment, drift in the properties of optical components (e.g., due to temperature changes), and variations in the laser source can cause shifts in the voxel position.

[0113]

[0112] Any effects of such shifts can be mitigated by forming predetermined sighting marks within the substrate. These sighting marks are sometimes called preambles.

[0114]

[0113] A sighting mark is a group of voxels arranged in a predetermined pattern. Generally, the pattern is a one-dimensional ("1D") or two-dimensional ("2D") pattern, but a three-dimensional pattern may also be used as an alternative. The size of the group is not particularly limited, provided that the number of voxels is sufficient to clearly identify the pattern when reading the optical data storage medium. For example, a 2D sighting mark may be at least 4 voxels wide × 4 voxels high.

[0115]

[0114] When processing the image of the optical data storage medium to restore the stored data, sight marks may be identified, and the positions of the voxels forming the sight marks may be determined. Based on the determined positions, corrections may be applied to compensate for positional variations and / or image distortions that occurred when writing the voxels.

[0116]

[0115] Location information may be used as input to a processing / decoding method for recovering data from an optical data storage medium. The processing / decoding method may include the use of a machine learning model.

[0117]

[0116] The sighting marks may be positioned, for example, along one or more edges of a sector of a voxel. Other arrangements are also conceivable. For example, the sighting marks may, alternatively or additionally, be positioned at the center of the sector.

[0118]

[0117] A sector is a two-dimensional group of voxels. In many implementations, a particular voxel layer in an optical data storage medium contains two or more sectors and is read using a reader with a field of view large enough to capture an image that includes the entire sector along with the edges of up to eight directly adjacent sectors.

[0119]

[0118] The given pattern may be a Barker sequence or a Frank-Zadoff-Chu sequence. Other patterns may also be used.

[0120]

[0119] The symbols forming the sighting marks may be selected to maximize the contrast between the symbols. This can allow for easier detection of the marks.

[0121]

[0120] A single optical data storage medium may have two or more different sighting marks. Adjacent sectors may be associated with different sighting marks. This can make it easier to distinguish sectors from one another.

[0122]

[0121] For example, two different sighting marks may be used, such that there are no sectors that share an edge with another sector having the same sighting mark, and the related sectors are arranged in a checkerboard pattern.

[0123]

[0122] Writing sighting marks to an optical data storage medium is particularly useful when the optical data storage medium is written to using a high-throughput method, such as the method described with reference to Figure 2. High-throughput methods can be sensitive to component drift. For example, operating the sample stage at high speed may involve a trade-off with positioning accuracy.

[0124]

[0123] Now, with reference to Figure 6, an exemplary laser writing system 600 useful for implementing the method described herein will be described. Figure 6 is a schematic block diagram of the laser writing system 600.

[0125]

[0124] An exemplary laser writing system 600 includes a laser source 610, a first beam splitter 630, a writing path 640, and a sample stage 660. The figure shows the laser writing system 600 in use with a substrate 620 mounted on the sample stage.

[0126]

[0125] The laser source 610 generates laser light pulses. Generally, laser pulses have a period t p It is generated at a constant repetition rate. For example, the repetition rate may be on the order of 10 MHz. The laser source 610 may be a femtosecond laser.

[0127]

[0126] The laser source 610 emits pulses of laser light toward the first beam splitter 630. The first beam splitter 630 splits each pulse into two or more beams. Each beam is directed toward its respective write path 640. By splitting the laser light into two or more beams, two or more instances of the method described herein can be performed in parallel (for example, enabling two or more scan lines to be written simultaneously).

[0128]

[0127] The figure shows a single writing path 640. The writing path 640 includes, in order, an intensity modulator 642, a second beam splitter 644, a polarization modulator 646, a scanner 648, and an objective lens 650.

[0129]

[0128] The intensity modulator 642 receives laser pulses from the first beam splitter 630 and adjusts the intensity of the laser pulses. The strength of a voxel may vary as a function of the amplitude of the laser pulses used to form the voxel, and the intensity modulator can make it possible to adjust this.

[0130]

[0129] The amplitude modulator may include an intensity attenuator and / or an amplifier.

[0131]

[0130] The intensity attenuator may include, for example, a combination of an acousto-optic deflector, a liquid crystal modulator, an electro-optic modulator and a polarizer, or a combination of a Pockels cell and a polarizer. These are examples of high-speed amplitude modulators. An amplitude modulator capable of controlling the inter-pulse energy is considered "high-speed".

[0132]

[0131] The intensity modulator 642 may be an acousto-optic deflector, or a combination of an electro-optic modulator and a polarizer.

[0133]

[0132] The laser pulse then reaches the second beam splitter 644. The second beam splitter 644 divides the pulse into a seed pulse and a data pulse, each having different intensities, as described above. The second beam splitter may be an electro-optic deflector, an acousto-optic deflector, or a polarization grating-based beam splitter.

[0134]

[0133] The second beam splitter 644 may alternatively comprise a combination of a passive beam splitter (e.g., selected from a non-polarization-sensitive beam splitter, a polarization-sensitive beam splitter, a diffractive optical element, and a spatial light modulator) combined with an active spatial modulator and an active amplitude modulator.

[0135]

[0134] The second beam splitter 644 is most preferably a beam splitter of the type described later with reference to Figure 7.

[0136]

[0135] At least the data pulse from the second beam splitter 644 passes through the polarization modulator 646. The polarization modulator modulates the polarization of the data pulse at the repetition rate of the laser.

[0137]

[0136] The polarization modulator 646 may be, for example, a polarization state generator. An exemplary polarization state generator comprises a linear polarizer and a quarter-wave plate arranged in series. Other examples of polarization state generators include Pockels cells, magneto-optical modulators, liquid crystals, rotating prisms, rotating periscopes, and the like.

[0138]

[0137] Since a specific polarization of the seed pulse is not required, the seed pulse may pass through the polarization modulator 646, or alternatively, the polarization modulator 646 may be bypassed. In an implementation where the seed pulse passes through the polarization modulator 636, the polarization modulator 646 modulates the polarization of both the data pulse and the seed pulse.

[0139]

[0138] The scanner 648 is located downstream of the polarization modulator 646. The scanner 648 may include a rotating polygon mirror, a microelectromechanical system ("MEMS") mirror, a galvanometer scanner, an electro-optical scanner, or an acousto-optical scanner.

[0140]

[0139] In an implementation where the seed pulse does not pass through the polarization modulator 646, the scanner 648 may align the seed pulse and the data pulse.

[0141]

[0140] The scanner 648 deflects the seed pulse and the data pulse. The deflection angle may be swept at a constant speed.

[0142]

[0141] The scanner 648 outputs seed pulses and data pulses to the objective lens 650. The objective lens 650 focuses the seed pulses and data pulses onto or within the substrate 320 to enable voxel formation, as shown with reference to Figure 2.

[0143]

[0142] The objective lens 650 may have a variable depth of field or focal position. In this case, voxels can be written at the variable depth position within the transparent substrate 620 without requiring movement of the transparent substrate in the z direction.

[0144]

[0143] A relay optical system may be placed between the scanner 648 and the objective lens 660. The relay optical system may include a scanning lens, a spherical lens, and a tube lens.

[0145]

[0144] The laser writing system further includes a sample stage 650 for holding the substrate 620. The sample stage may be a translation stage that moves the substrate 620 perpendicular to the beam direction and perpendicular to the beam scanning direction.

[0146]

[0145] Various modifications may be made to this exemplary write head.

[0147]

[0146] The first beam splitter 620 is optional and may be omitted in implementations where the parallelization of this method is not used.

[0148]

[0147] When parallelization is used, there is no particular upper limit on the number of write paths. If three or more write paths are used, the first beam splitter 620 may be replaced with two or more beam splitters arranged in series. For example, the system may include 1 to 5 write paths.

[0149]

[0148] In this example, the scanner 648 and the objective lens 650 are components of the writing path. Alternatively, the scanner 648 and the objective lens 650 may be shared among several writing paths. In such an implementation, the beams may be recombined after polarization modulation.

[0150]

[0149] The intensity modulator 642 is optional and may be omitted.

[0151]

[0150] In principle, the writing path may include separate optical components for the seed pulse and the data pulse. It may be advantageous for the seed pulse and the data pulse to follow the same optical path in order to enable more precise control over the distance between the seed pulse and the data pulse.

[0152]

[0151] The writing head may include one or more additional optical relay components (e.g., one or more lenses) positioned between the second beam splitter 644 and the scanner 648 so that seed pulses and data pulses reach the same area of ​​the scanner.

[0153]

[0152] The scanner 648 may be omitted. Instead, the positions of the data pulses and seed pulses in the transparent substrate can be controlled solely by moving the sample stage.

[0154]

[0153] An exemplary sample stage is a translation stage. Alternatively or additionally, the sample stage may rotate the transparent substrate.

[0155]

[0154] In the illustrated example, both the seed pulse and the data pulse are generated using a single laser source. In other implementations, the seed pulse and the data pulse may be generated by their respective laser sources. In such implementations, the second beam splitter may be omitted. If two or more laser sources are used, the laser sources may be laser sources of different wavelengths.

[0156]

[0155] Now, an exemplary beam splitter 700 will be described with reference to Figure 7. Figure 7 is an annotated schematic diagram of the beam splitter 700.

[0157]

[0156] The beam splitter 700 comprises a retardation plate 710 arranged in series, a first polarization-selective diffractive optic 720, and a second polarization-selective diffractive optic 730.

[0158]

[0157] Polarization-selective diffraction optical systems are sometimes alternatively called "geometric phase optical elements" or "geometric phase elements."

[0159]

[0158] In this example, the phase difference plate 710 is a quarter-wave plate, the first polarization-selective diffraction optical system 720 is a first polarization grating, and the second polarization-selective diffraction optical system is a second polarization grating.

[0160]

[0159] The phase difference plate 710, the first polarizing grating 720, and the second polarizing grating 730 are arranged in series.

[0161]

[0160] During use, the polarized input light LP (generated, for example, by the laser source 610 of the laser writing system 600) reaches the quarter-wave plate 710. The input light may be linearly polarized, elliptically polarized, or circularly polarized. For simplicity of explanation, in the following description, the input light will be assumed to be linearly polarized.

[0162]

[0161] When linearly polarized light passes through the phase difference plate 710, it becomes elliptically polarized light EP. By adjusting the angle A of the quarter-wave plate 710, any ellipticity can be obtained.

[0163]

[0162] Subsequently, the elliptic polarization EP reaches the first polarization grating 720. The first polarization grating 720 splits the elliptic polarization into a left circularly polarized beam and a right circularly polarized beam. The relative amplitude of these two beams is determined by the ellipticity of the elliptic polarization EP. Thus, the polarization grating-based beam splitter 700 is a convenient means for generating seed pulses and data pulses with the intended amplitude ratio in the method shown in Figure 2.

[0164]

[0163] Subsequently, the left circularly polarized beam and the right circularly polarized beam reach the second polarizing grating 730. Since these beams are circularly polarized, each beam is deflected in a single direction by the second polarizing grating.

[0165]

[0164] Angle Θ between beam LHC and beam RHC OUT This can be changed by altering the angle ≠1 of the first polarizing grating relative to the angle □2 of the second polarizing grating.

[0166]

[0165] More specifically, the polarization angle can be changed by changing the relative orientation of the lattice vectors of the first and second polarization gratings. The splitting angle is minimized when the lattice vectors are parallel and maximized when the lattice vectors are antiparallel.

[0167]

[0166] Here, we will explain the mathematical process of beam splitting.

[0168]

[0167] Elliptic polarization can be represented by Jones vectors:

number

[0169]

[0168] Right-hand circular polarization and left-hand circular polarization are expressed as follows:

number

[0170]

[0169] Equation A1.1 can be expressed as a linear combination of the Jones vectors of two circularly polarized signals:

number

[0171]

[0170] The splitting angle can be adjusted by changing the relative orientation of the polarization gratings 720 and 730 using the beam splitter 700.

[0172]

[0171] For illustrative purposes, first consider the case where the input light EP is right-circularly polarized. This light is deflected in only one direction by the first polarizing grating 720 and converted to left-circular polarization. The spatial phase change caused by the first polarizing grating is as follows:

number

[0173]

[0172] Due to symmetry, the second polarizing grating 730 causes the following phase change:

number

[0174]

[0173] Therefore, the total phase change for right-circularly polarized light passing through the beam splitter is as follows:

number

[0175]

[0174] When the deflection angles due to the two PGs are the same (q=q1=q2), equation A2.3 can be simplified as follows:

number

[0176]

[0175] The total phase change of left-circularly polarized light passing through the beam splitter is as follows:

number

[0177]

[0176] Elliptic polarization can be treated as a combination of left-circular polarization and right-circular polarization. According to equation A1.3, elliptic polarization of ellipticity c has two circularly polarized light with the following energy ratio: (cosχ-sinχ) 2 :(cosχ-sinχ) 2 (eq.A2.6)

[0178]

[0177] Therefore, the left circularly polarized component and the right circularly polarized component undergo different phase changes (Equations A2.4 and A2.5). Due to these phase changes, the light is split at the following angles:

number

[0179]

[0178] The slope of the dividing plane from the xy-plane is as follows:

number

[0180]

[0179] Equations A2.7 and A2.8 suggest that the division angle can be changed from 0 to approximately 4q by changing the relative orientation (j1-j2) between the two polarization gratings from 0 degrees (parallel) to 180 degrees (antiparallel), and the division plane is determined by the sum of these orientations (j1+j2). In addition, equation A2.6 means that the amplitude ratio of the two beams can be changed by changing the ellipticity of the input light.

[0181]

[0180] Various modifications may be made to the polarizing grating-based beam splitter described.

[0182]

[0181] For example, the quarter wave plate 710 may be replaced with a wave plate having a different retardance, such as a third wave plate. A third wave plate can provide a smaller ratio between the amplitudes of the beams output by the beam splitter.

[0183]

[0182] A polarizing hologram or polarizing lens may be used as an alternative to the polarizing grating. For example, if the polarizing grating is replaced with a polarizing lens, the splitter may produce two beams with different divergences. The beams with different divergences may be combined, for example, using the focusing optical system 650 of the laser writing system 600 to produce a multifocal beam (i.e., two focused beams in the direction of light propagation).

[0184]

[0183] Two or more beam splitters of the described type may be arranged in series to generate multiple beams having a variable propagation direction and a variable amplitude ratio. [Examples]

[0185] Example 1

[0184] A birefringent voxel layer was written onto a silica glass medium using the method shown in Figure 2.

[0186]

[0185] The writing was performed using a laser writing system of the type described with reference to Figure 6. The laser source of the laser writing system was a femtosecond laser. A polarization modulator was synchronized with the laser. The laser writing system further included a high-speed beam scanner, an XYZ translation stage, a high-speed attenuator, and a beam splitter.

[0187]

[0186] The data was encoded into eight polarization azimuth angle values, i.e., 3 bits per voxel. Each voxel was written using one seed pulse and one data pulse with an intensity ratio of approximately 0.6 to 0.7.

[0188]

[0187] The lateral pitch was approximately 0.5 × 0.5 microns. The spatial separation between the seed beam and the data beam within the medium was approximately 2 microns. The beam scanning speed within the medium was approximately 5 m / s.

[0189]

[0188] A peak throughput of approximately 10 million voxels per second per beamline was achieved.

[0190] Example 2

[0189] Two types of optical data storage voxels were written using the comparative hot writing method described in Lei et al., Optica Vol.8, Issue 11, pp.1365-1371 (2021) and the method shown in Figure 2, respectively. Scanning electron microscope images of the voxels in the medium are shown in Figures 8A and 8B, respectively.

[0191]

[0190] The voxels obtained by the comparative method were found to contain a central ridge and asymmetrical, elongated cracks on both sides of the central ridge (Figure 8A). In contrast, the voxels obtained by the method in Figure 2 had an elliptical shape (Figure 8B).

[0192]

[0191] Although not bound by theory, the difference in shape may be a result of improved alignment between the seed pulse and data pulse achieved by the method in Figure 2. The asymmetry of the voxels obtained using the comparative hot lighting method is thought to be a result of the seed pulse and data pulse arriving at slightly different positions, which is unavoidable in the hot lighting process.

[0193] Example 3

[0192] Data was written to an optical storage medium using the cold writing technique. Various data pulse counts per voxel were used. The storage capacity of the medium was measured by the number of bits per voxel. The results are shown in Figure 9.

[0194]

[0193] It was found that the difference in polarization between the seed pulse and the data pulse did not significantly affect the quality of the voxel (Figure 9, the two data points on the far left). The azimuth angle of the birefringent voxel was determined by the polarization of one or more data pulses and did not respond to the polarization of the seed pulse.

[0195]

[0194] When two or more data pulses were used per voxel, i.e., when the total number of pulses per voxel exceeded two, a slight improvement in quality was observed when all data pulses had the same polarization. This improvement was not significant enough to be considered statistically significant. When multiple data pulses had different polarizations, the quality deteriorated.

[0196]

[0195] Therefore, Figure 9 demonstrates that the quality of birefringent voxels is not affected by seed polarization, and that there is no significant advantage to using more than two data pulses per voxel.

[0197] Example 4

[0196] The effect of walk-off distance on data capacity per voxel was investigated for the comparative cold writing process and the process according to Figure 2.

[0198]

[0197] Walk-off exceeding ±20 nm was found to cause observable degradation of birefringent voxels in the cold lighting process (Figure 10A).

[0199]

[0198] When the process according to Figure 2 was used (Figure 10B), a similar effect was observed when the seed pulse and data pulse were separated by a non-integer multiple of the voxel pitch.

[0200]

[0199] These results demonstrate that, in order to minimize the walk-off distance and thereby maximize data capacity, it is most preferable to set the interval between the seed pulse and the data pulse to an integer multiple of the voxel pitch.

[0201] Example 5

[0200] A beam splitter of the type described with reference to Figure 7 was constructed. In this exemplary device, both polarization gratings had the same grating period.

[0202]

[0201] The orientation of the polarizing grating ("PG") was changed. More specifically, the difference between their orientations (φ PG1 -φ PG2 ) while keeping their sums the same (φ PG1 +φ PG2 The (constant) distance was varied. A beam profiler was used to monitor the distance between the two output beams. Based on the measured distance, the separation angle was calculated. The results are shown in Figure 11.

[0203]

[0202] The maximum separation angle in this measurement was 3.7 mrad, which was twice the separation angle achieved using a single PG. The separation angle followed the following formula:

number

[0204]

[0203] Further experiments were conducted to demonstrate that the relative amplitude of the two beams output by the beam splitter can be changed by adjusting the angle of the quarter-wave plate ("QWP"). Linear polarization was used as the input. The results are shown in Figure 12.

[0205]

[0204] When the polarization before the QWP is linear, it was found that the intensity ratio follows the sine of the angle of the QWP:

number

[0206]

[0205] It should be understood that the above embodiments are described merely as examples.

[0207]

[0206] More generally, according to one aspect disclosed herein, a method is provided for forming birefringent voxels in a transparent substrate. This method is i) A step of simultaneously generating a first seed pulse and a first data pulse, wherein the first seed pulse and the first data pulse are spatially separated laser pulses, and the first seed pulse has an amplitude different from the amplitude of the first data pulse. ii) A step of focusing a first seed pulse to a first seed location in the substrate and a data pulse to a first data location in the substrate, wherein the first seed location and the first data location are separated by a predetermined distance along the scanning path, and the first seed location is located before the first data location in the scanning path. iii) A step of simultaneously generating a second seed pulse and a second data pulse after steps i) and ii), wherein the second seed pulse and the second data pulse are spatially separated laser pulses, and the second seed pulse has an amplitude different from that of the second data pulse. iv) A step of focusing a second seed pulse to a second seed location in the substrate and a data pulse to a second data location in the substrate, wherein the second seed location is separated from the second data location by a predetermined distance along the scanning path, the second seed location is located in front of the first seed location and the second data location in the scanning path, the second data location is the same as the first seed location, and thereby the formation of a birefringent voxel. Includes.

[0208]

[0207] Related methods are, i) A step of simultaneously generating a seed pulse and a data pulse, wherein the seed pulse and the data pulse are spatially separated laser pulses, and the seed pulse has an amplitude different from the amplitude of the data pulse. ii) A step of focusing seed pulses and data pulses to respective locations within the substrate, wherein these locations are separated by a predetermined distance along the scanning path, and the location of the seed pulses is located before the location of the data pulses in the scanning path. iii) A step of repeating steps i) and ii), wherein between iterations, the locations of the seed pulse and the data pulse are incremented along the scan path, such that the location of the seed pulse in the previous iteration is the same as the location of the data pulse in the subsequent iteration, thereby resulting in the formation of a birefringent voxel at that location. This includes the following. It will be understood that various optional features listed below can be incorporated into the implementation of either method.

[0209]

[0208] Generally, the amplitude of the seed pulse is larger than the amplitude of the data pulse.

[0210]

[0209] The substrate may be a transparent substrate. "Transparent" means that it transmits light of one or more wavelengths of the seed pulse and data pulse. If the substrate is a transparent substrate, the locations of the seed pulse and data pulse may be inside the body of the substrate or on the surface of the substrate.

[0211]

[0210] Alternatively, the substrate may be an opaque substrate. In this type of mounting configuration, the location is on the surface of the substrate.

[0212]

[0211] Generating seed pulses and data pulses may include generating source laser pulses and splitting the source laser pulses using a beam splitter. This can enable more efficient use of the laser.

[0213]

[0212] Alternatively, the seed pulse and data pulse may be generated by their respective laser sources. This makes it possible for the seed pulse and data pulse to have different wavelengths.

[0214]

[0213] The beam splitter may include a phase difference plate, a first polarization-selective diffraction optical system located downstream of the phase difference plate along the optical path, and a second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path. Such a beam splitter can enable control of the amplitude of the source pulse and data pulse, as well as a predetermined distance.

[0215]

[0214] The phase difference plate may be a quarter-wave plate.

[0216]

[0215] Examples of polarization-selective diffraction optical systems include polarizing gratings, polarizing holograms, and polarizing lenses. The first and second polarization-selective diffraction optical systems may be the same or different. Specifically, the first polarizer may be a first polarizing grating, and the second polarizer may be a second polarizing grating.

[0217]

[0216] The locations of the seed pulse and data pulse may be incremented between iterations by a fraction of a non-zero integer of a predetermined distance. In other words, the locations of the seed pulse and data pulse may be incremented by a non-zero integer multiple of the voxel pitch. This allows for more reliable alignment of the data pulse of a later iteration with respect to the seed pulse of the previous iteration.

[0218]

[0217] The divisors of the non-zero integer may be, for example, at least 2. By changing the interval between the seed pulse and the data pulse, the time delay between the arrival of the seed pulse and the arrival of the data pulse at any particular location is changed. By providing such a time delay, thermal damage to the transparent substrate can be avoided.

[0219]

[0218] The predetermined distance may be twice or more the wavelength of the seed pulse and the data pulse. This can reduce or avoid crosstalk between the seed pulse and the data pulse. A smaller predetermined distance may be used.

[0220]

[0219] The method may further include modulating the polarization of the data pulses. That is, the polarization of different iterations of the data pulses may be different. Modulating the polarization can enable the storage of higher density data, since each different polarization can represent a different data symbol.

[0221]

[0220] The seed pulse and data pulse may share a common path through the scanning optical system and the delivery optical system. This allows for more reliable positioning of the seed pulse and data pulse.

[0222]

[0221] By repeating steps i) and ii), multiple birefringent voxels can be formed, which represent sighting marks. The sighting marks are most commonly one-dimensional or two-dimensional, but three-dimensional sighting marks may also be used. The sighting marks can be recognized by a reader when recovering data from an optical data storage medium formed by this method. This allows the reader to compensate for positional inaccuracies during the writing process.

[0223]

[0222] This method may be parallelized. Two or more instances of this method may be executed simultaneously. Two or more instances of step i) and step ii) may be executed in parallel simultaneously. This can improve throughput.

[0224]

[0223] Another embodiment provides an optical data storage medium obtained by the present method. As described herein with reference to Figures 8A and 8B, voxels written by the present method have a different structure from voxels produced by the comparative hot writing process.

[0225]

[0224] The optical data storage medium may be a transparent optical data storage medium comprising a transparent substrate and birefringent voxels embedded in the transparent substrate. The birefringent voxels may be in the form of elliptical voids having a diameter of 1 to 200 nm, optionally in the range of 50 to 200 nm, measured along the semi-major axis of the ellipse.

[0226]

[0225] The optical data storage medium may include voxels arranged along the scan line. The first end region of the scan line may include at least one weak modification obtained by applying a data pulse to a blank region of the substrate. The second end region of the scan line may include one or more non-birefringent seed modifications. A data encoding region including one or more birefringent voxels may be located between the first end region and the second end region.

[0227]

[0226] An optical data storage medium may comprise a layer having two or more such scan lines. The pitch between voxels within one scan line may differ from the pitch between adjacent scan lines.

[0228]

[0227] An optical data storage medium may comprise two or more such layers arranged in a stack. The number of layers is not particularly limited, provided that the data is recoverable by a suitable reading system. For example, an optical data storage medium may comprise 2 to 100 or more birefringent voxel layers.

[0229]

[0228] The optical data storage medium may include a sighting mark, and optionally a plurality of birefringent voxels arranged as a two-dimensional sighting mark. The two-dimensional sighting mark may be used to assist in the reconstruction of data encoded by the voxels. For example, the position of the voxels within the two-dimensional sighting mark may be used to determine one or more correction parameters to compensate for errors in voxel positioning.

[0230]

[0229] The birefringent voxels may be arranged in multiple sectors. Each sector may be associated with its respective sighting mark, for example, located on the edge of the sector.

[0231]

[0230] The transparent substrate may be a glass substrate, optionally a silica glass substrate, or a quartz glass substrate. Glass has excellent chemical, thermal, and physical stability, which allows for the storage of data over very long periods.

[0232]

[0231] Another embodiment provides a laser writing system useful for implementing the present method. The laser writing system comprises a laser source for emitting laser pulses and a writing path for receiving input laser pulses generated from the laser source and for focusing seed pulses and data pulses to respective locations on the substrate.

[0233]

[0232] The writing path comprises a beam splitter for splitting a laser pulse into a seed pulse and a data pulse, wherein the seed pulse and the data pulse are spatially separated and the seed pulse has an amplitude different from (e.g., larger than) the amplitude of the data pulse, and an objective lens for focusing the seed pulse and the data pulse to different locations on the substrate, wherein the objective lens is located downstream of the beam splitter in the optical path.

[0234]

[0233] The writing path may further include a scanning optical system disposed, for example, between a beam splitter and an objective lens. The scanning optical system can change the locations of the seed pulse and the data pulse.

[0235]

[0234] The laser writing system may further include a controller configured to control the writing head to implement the methods as described herein, or may be operably linked to such a controller. The nature of the controller is not particularly limited.

[0236]

[0235] The controller may include a dedicated hardware circuit, for example, an application specific integrated circuit (ASIC). The controller may include one or more processors and a non - transient computer - readable data storage that stores instructions that, when executed by the processor, cause the writing head to implement the methods as described herein. A combination of a dedicated hardware circuit and one or more processors may be used.

[0237] 【0,236】The beam splitter may include a retardation plate, a first polarization - selective diffractive optical system located downstream of the retardation plate along the optical path, and a second polarization - selective diffractive optical system located downstream of the first polarization - selective diffractive optical system along the optical path. Such a beam splitter provides a convenient means for generating a seed pulse and a data pulse having different amplitudes and a predetermined interval.

[0238]

[0237] The retardation plate may be a quarter - wave plate.

[0239] [[ID= 19]]

[0238] Examples of polarization-selective diffraction optical systems include polarizing gratings, polarizing holograms, and polarizing lenses. The first and second polarization-selective diffraction optical systems may be the same or different. Specifically, the first polarization-selective diffraction optical system may be a first polarizing grating, and the second polarization-selective diffraction optical system may be a second polarizing grating.

[0240]

[0239] The beam splitter may be configured to allow relative rotation of the first and second polarization-selective diffraction optical systems. This allows for control over the interval between the seed pulse and the data pulse.

[0241]

[0240] The phase difference plate may be adjustable. By adjusting the phase difference plate, the ellipticity of the light reaching the first polarization-selective diffraction optical system can be changed, and as a result, the amplitude ratio of the seed pulse and the data pulse can be changed.

[0242]

[0241] The laser writing system may further include a polarization modulator for modulating the polarization of the data pulse. Modulating the polarization of the data pulse makes it possible to write voxels with different birefringences onto a transparent substrate. The difference in birefringence (e.g., azimuthal angle) can be used to encode the data.

[0243]

[0242] The laser writing system may include two or more writing paths. By providing two or more writing paths, it is possible to enable two or more instances of the method provided herein to be executed simultaneously.

[0244]

[0243] For example, the laser writing system may include a further beam splitter located downstream of the laser source and upstream of the writing path. The second beam splitter may provide input pulses to each of two or more writing paths.

[0245]

[0244] Another exemplary laser writing system comprises a pulsed laser source, a beam splitter located downstream of the pulsed laser source along the optical path, and a focusing optical system located downstream of the beam splitter along the optical path. As can be understood, this laser writing system may include any of the features of the systems described above. For example, the beam splitter may comprise a phase difference plate, a first polarization-selective diffraction optical system located downstream of the quarter-wave plate along the optical path, and a second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path.

[0246]

[0245] In another embodiment, a beam splitter is provided. The beam splitter comprises a phase difference plate, a first polarization-selective diffraction optical system located downstream of the phase difference plate along the optical path, and a second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path. This beam splitter is useful for splitting a polarized beam. As described above, such a beam splitter allows control over both the relative amplitude and angular spacing of the split beams.

[0247]

[0246] The phase difference plate may be a quarter-wave plate.

[0248]

[0247] Examples of polarization-selective diffraction optical systems include polarizing gratings, polarizing holograms, and polarizing lenses. The first and second polarization-selective diffraction optical systems may be the same or different. Specifically, the first polarization-selective diffraction optical system may be a first polarizing grating, and the second polarization-selective diffraction optical system may be a second polarizing grating.

[0249]

[0248] The first and second polarization-selective diffraction optical systems may be rotatable relative to each other. This makes it possible to change the angular spacing between the split beams.

[0250]

[0249] The phase difference plate may be adjustable, for example, rotatable. By adjusting the phase difference plate, the ellipticity of the polarization reaching the first polarization-selective diffraction optical system can be adjusted, and as a result, the relative amplitude of the split beams generated by the beam splitter changes.

[0251]

[0250] This disclosure provides the following terms: Clause 1. To simultaneously generate a first seed pulse and a first data pulse, wherein the first seed pulse and the first data pulse are spatially separated laser pulses, and the amplitude of the first seed pulse is different from the amplitude of the first data pulse. The method involves focusing a first seed pulse to a first seed location on a substrate and focusing a data pulse to a first data location on a substrate, wherein the first seed location and the first data location are separated by a predetermined distance along the scanning path, and the first seed location is located before the first data location in the scanning path. The method involves focusing a first seed pulse to a first seed location and a data pulse to a first data location, and then simultaneously generating a second seed pulse and a second data pulse, wherein the second seed pulse and the second data pulse are spatially separated laser pulses, and the amplitude of the second seed pulse is different from the amplitude of the second data pulse. The method of focusing a second seed pulse to a second seed location in the substrate and a data pulse to a second data location in the substrate, wherein the second seed location is separated from the second data location by a predetermined distance along the scanning path, the second seed location is located before the first seed location and the second data location in the scanning path, the second data location is the same as the first seed location, and thereby the formation of a birefringent voxel is achieved. Methods that include... Clause 2. The method according to Clause 1, wherein the amplitude of the first seed pulse is greater than the amplitude of the first data pulse. Clause 3. The method according to Clause 1 or 2, wherein the substrate is a transparent substrate. Clause 4. The method of any one of the preceding clauses, wherein generating a first seed pulse and a first data pulse includes generating a source laser pulse and splitting the source laser pulse using a beam splitter. Clause 5. The beam splitter, Phase difference plate and A first polarization-selective diffraction optical system located downstream of the phase difference plate along the optical path, A second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path, The method described in Clause 4, comprising: Clause 6. The method according to Clause 5, wherein the phase difference plate is a quarter-wave plate. Clause 7. The method according to Clause 5 or 6, wherein each of the first polarization-selective diffraction optical system and the second polarization-selective diffraction optical system is independently selected from a polarizing grating, a polarizing hologram, and a polarizing lens. Clause 8. After focusing the first seed pulse to the first seed location and the data pulse to the first data location, and before focusing the second seed pulse to the second seed location and the data pulse to the second data location, a) Simultaneously generating an intermediate seed pulse and an intermediate data pulse, wherein the intermediate seed pulse and the intermediate data pulse are spatially separated laser pulses having different amplitudes. b) Focusing intermediate seed pulses and intermediate data pulses to intermediate seed locations and intermediate data locations within the substrate, respectively, such that the intermediate seed locations and intermediate data locations are separated by a predetermined distance, and the intermediate seed locations are located before the intermediate data locations in the scanning path. The method of any one of the preceding clauses, further comprising performing at least one iteration of the method. Clause 9. The method according to Clause 8, wherein adjacent seed locations are separated between iterations by a divisor of a non-zero integer of a given distance. Clause 10. The method according to Clause 9, wherein the number of non-zero integer divisors is at least 2. Clause 11. The method according to any one of the preceding clauses, wherein the predetermined distance is at least twice the wavelength of the seed pulse and the data pulse. Clause 12. The method according to any one of the preceding clauses, further comprising modulating the polarization of the first data pulse and the second data pulse. Clause 13. The method according to any one of the preceding clauses, wherein the first seed pulse and the first data pulse share a common path passing through a scanning optical system and a delivery optical system. Clause 14. Generating a third seed pulse and a third data pulse simultaneously with generating the first seed pulse and the first data pulse, wherein the third seed pulse and the third data pulse are spatially separated laser pulses, and the amplitude of the third seed pulse is different from the amplitude of the third data pulse, and Focusing the first seed pulse at a first seed location and the first data pulse at a first data location, and simultaneously focusing the third seed pulse at a third seed location within the substrate and the third data pulse at a third data location within the substrate, wherein the third seed location and the third data location are separated by a predetermined distance along a further scanning path, and the third seed location is in front of the third data location on the further scanning path, and The method according to any one of the preceding clauses, further comprising. Clause 15. i) Simultaneously generating a seed pulse and a data pulse, wherein the seed pulse and the data pulse are spatially separated laser pulses, and the seed pulse has an amplitude different from the amplitude of the data pulse, and ii) Focusing the seed pulse and the data pulse at respective locations within the substrate, wherein these locations are separated by a predetermined distance along a scanning path, and the location of the seed pulse is in front of the location of the data pulse in the scanning path. iii) A step of repeating steps i) and ii), wherein between repetitions, the locations of the seed pulse and the data pulse are incremented along the scan path, such that the location of the seed pulse in the previous repetition is the same as the location of the data pulse in the later repetition, thereby resulting in the formation of a birefringent voxel at that location. Methods that include... Clause 16. The method according to Clause 15, wherein the amplitude of the seed pulse is greater than the amplitude of the data pulse. Clause 17. The method according to Clause 15 or 16, wherein the substrate is a transparent substrate. Clause 18. The method according to any one of Clauses 15 to 17, wherein generating seed pulses and data pulses includes generating source laser pulses and splitting source laser pulses using a beam splitter. Clause 19. The beam splitter, Phase difference plate and A first polarization-selective diffraction optical system located downstream of the phase difference plate along the optical path, A second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path, The method described in Clause 18, comprising: Clause 20. The method according to Clause 19, wherein the phase difference plate is a quarter-wave plate. Clause 21. The method according to Clause 19 or 20, wherein each of the first polarization-selective diffraction optical system and the second polarization-selective diffraction optical system is independently selected from a polarizing grating, a polarizing hologram, and a polarizing lens. Clause 22. The method according to any one of Clauses 15 to 21, wherein the location of the seed pulse and the location of the data pulse are incremented by a non-zero integer fraction of a predetermined distance between iterations. Clause 23. The method described in Clause 22, wherein the number of non-zero integer divisors is at least 2. Clause 24. The method according to any one of Clauses 15 to 23, wherein the specified distance is at least twice the wavelength of the seed pulse and the data pulse. Clause 25. The method described in any one of Clauses 15 to 24, further comprising modulating the polarization of a data pulse. Clause 26. The method according to any one of Clauses 15 to 25, wherein the seed pulse and data pulse share a common path through the scanning optical system and the delivery optical system. Clause 27. The method according to any one of Clauses 15 to 26, wherein the formation of multiple birefringent voxels is brought about by repeating steps i) and ii), the multiple birefringent voxels representing a sighting mark. Clause 28. The method described in any one of Clauses 15 to 27, wherein two or more instances of step i) and step ii) are performed concurrently. Clause 29. An optical data storage medium obtained by the method described in any one of the preceding clauses. Clause 30. Transparent substrate and, A birefringent voxel embedded in a transparent substrate, Equipped with, The optical data storage medium according to Clause 29, wherein the birefringent voxels are in the form of elliptical voids with a major axis having a diameter in the range of 1 to 200 nm. Clause 31. An optical data storage medium as described in Clause 30, wherein the transparent substrate is a glass substrate. Clause 32. An optical data storage medium according to Clause 30 or 31, comprising multiple birefringent voxel layers. Clause 33. An optical data storage medium as described in any one of Clauses 30 to 32, comprising multiple birefringent voxels arranged as sighting marks. Clause 34. A transparent optical data storage medium as described in Clause 33, wherein the sighting mark is a two-dimensional sighting mark. Clause 35. A laser source for emitting laser pulses, A writing path for receiving input laser pulses from a laser source and focusing seed pulses and data pulses to their respective locations on the substrate, A laser writing system comprising, The writing path is A beam splitter for splitting an input pulse into a seed pulse and a data pulse, wherein the seed pulse and the data pulse are spatially separated, and the seed pulse has an amplitude different from that of the data pulse. An objective lens for focusing seed pulses and data pulses to different locations within a transparent substrate, wherein the objective lens is located downstream of the beam splitter on the writing path, A laser writing system equipped with the following features. Clause 36. The laser writing system according to Clause 35, wherein the beam splitter is configured to split an input pulse into a seed pulse and a data pulse, the seed pulse having an amplitude greater than the amplitude of the data pulse. Clause 37. The laser writing system according to Clause 35 or 36, further comprising a scanning optical system for changing the location of seed pulses and data pulses. Article 38. The beam splitter, Phase difference plate and A first polarization-selective diffraction optical system located downstream of the quarter-wave plate along the optical path, A second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path, A laser writing system as described in any one of clauses 35 to 37, comprising: Clause 39. The laser writing system according to Clause 38, wherein the phase difference plate is a quarter-wave plate. Clause 40. The laser writing system according to Clause 38 or 39, wherein each of the first polarization-selective diffraction optical system and the second polarization-selective diffraction optical system is independently selected from a polarizing grating, a polarizing hologram, and a polarizing lens. Clause 41. A laser writing system according to any one of Clauses 38 to 40, wherein the first and second polarization-selective diffraction optical systems are rotatable relative to each other. Clause 42. A laser writing system according to any one of Clauses 38 to 41, further comprising a polarization modulator for modulating the polarization of a data pulse. A write head according to any one of the clauses 38 to 42, comprising two or more write paths and a beam splitter positioned between a laser source and two or more write paths, wherein the beam splitter is configured to split a laser pulse to generate an input pulse for each write path. Clause 44. Phase difference plate and, A first polarization-selective diffraction optical system located downstream of the phase difference plate along the optical path, A second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path, A beam splitter equipped with [a specific feature]. Clause 45. A beam splitter as described in Clause 44, wherein the phase difference plate is a quarter-wave plate. Clause 46. The beam splitter according to Clause 44 or 45, wherein each of the first polarization-selective diffraction optical system and the second polarization-selective diffraction optical system is independently selected from a polarizing grating, a polarizing hologram, and a polarizing lens. Clause 47. A beam splitter according to any one of Clauses 44 to 46, configured to allow relative rotation of a phase difference plate, a first polarization-selective diffraction optical system, and a second polarization-selective diffraction optical system. Clause 48. Pulsed laser source and, A beam splitter positioned downstream of a pulsed laser source along the optical path, wherein the beam splitter is Phase difference plate and A first polarization-selective diffraction optical system located downstream of the quarter-wave plate along the optical path, A second polarization-selective diffraction optical system located downstream of the first polarization-selective diffraction optical system along the optical path, A beam splitter equipped with, A laser writing system comprising, A laser writing system further comprising a focusing optical system located downstream of the beam splitter in the optical path. Clause 49. The laser writing system according to Clause 48, wherein each of the first polarization-selective diffraction optical system and the second polarization-selective diffraction optical system is independently selected from a polarization grating, a polarization hologram, and a polarization lens. Clause 50. The laser writing system according to Clause 48 or 49, wherein at least one of a phase difference plate, a first polarization-selective diffraction optical system, and a second polarization-selective diffraction optical system is rotatably mounted.

[0252]

[0251] With the disclosure of this specification, those skilled in the art may see other variations or uses of the disclosed techniques. The scope of this disclosure is not limited by the embodiments described, but is limited only by the appended claims.

Claims

1. First seed pulse (S ０ ) and the first data pulse (D ０ (201) simultaneously generate the first seed pulse (S ０ ) and the first data pulse (D ０ ) is a spatially separated laser pulse, and the first seed pulse (S ０ The amplitude of the first data pulse (D ０ The amplitude of ) is different from the generation (201), The first seed pulse (S ０ ) is focused on a first seed location (340(0)) within the substrate (320), and the first data pulse (D ０ ) is focused on a first data location (330(0)) within the substrate (320), wherein the first seed location (340(0)) and the first data location (330(0)) are separated by a predetermined distance (jp ｆ ) along a scanning path (V ｖ ), and the first seed location (340(0)) is in front of the first data location (340(0)) in the scanning path (V ｆ ), the focusing; The first seed pulse (S ０ ) is focused to the first seed location (340(0)), and the first data pulse (D ０ After focusing the second seed pulse (S) to the first data location (330(0)), ２ ) and the second data pulse (D ２ The means of simultaneously generating the second seed pulse (S ２ ) and the second data pulse (D ２ ) is a spatially separated laser pulse, and the second seed pulse (S ２ The amplitude of the second data pulse (D ２ The amplitude of ) is different from the generation and The second seed pulse (S) ２ ) is focused to a second seed location (340(2)) in the substrate, and the second data pulse (D ２ The method involves focusing the second seed location (340(2)) on the substrate, where the scanning path (V ｆ ) along the predetermined distance (jp ｖ ) is separated from the second data location, and the second seed location (340(2)) is the scan path (V ｆ ) is located before the first seed location (340(0)) and the second data location, and the second data location is the same as the first seed location (340(0)), thereby resulting in the formation of a birefringent voxel (350(0)), and focusing, Methods that include... The distance along the scanning path, the second seed location is before the first seed location and the second data location in the scanning path, and the second data location is the same as the first seed location, thereby resulting in the formation of a birefringent voxel.

2. The first seed pulse (S ０ The amplitude of the first data pulse (D ０ The method according to claim 1, wherein the amplitude is greater than the amplitude of the above.

3. The method according to claim 1 or 2, wherein the substrate (320) is a transparent substrate.

4. The first seed pulse (S ０ ) and the first data pulse (D ０ The method according to any one of claims 1 to 3, wherein generating a source laser pulse and splitting the source laser pulse using a beam splitter (644).

5. The beam splitter, Phase difference plate (710), A first polarization-selective diffraction optical system (720) located downstream of the phase difference plate (710) along the optical path, A second polarization-selective diffraction optical system (730) is located downstream of the first polarization-selective diffraction optical system (720) along the optical path, The method according to claim 4, comprising:

6. The method according to claim 5, wherein the phase difference plate (710) is a quarter-wave plate.

7. The method according to claim 5 or 6, wherein the first polarization-selective diffraction optical system (720) includes one of a polarizing grating, a polarizing hologram, or a polarizing lens, and the second polarization-selective diffraction optical system (730) includes one of a polarizing grating, a polarizing hologram, or a polarizing lens.

8. The first seed pulse (S ０ After focusing the first seed pulse (D0) to the first seed location (340(0)) and focusing the first data pulse (D0) to the first data location (330(0)), and before focusing the second seed pulse (S2) to the second seed location (340(2)) and focusing the second data pulse to the second data location, The method involves simultaneously generating an intermediate seed pulse and an intermediate data pulse, wherein the intermediate seed pulse and the intermediate data pulse are spatially separated laser pulses having different amplitudes. The intermediate seed pulse and the intermediate data pulse are focused to the intermediate seed location and the intermediate data location within the substrate, respectively, wherein the intermediate seed location and the intermediate data location are located at a predetermined distance (jp ｖ ) is separated, and the intermediate seed location is before the intermediate data location in the scan path, to be focused, The method according to any one of claims 1 to 7, further comprising performing at least one iteration of the method.

9. The adjacent seed locations (330(0), 330(1)) are within the predetermined distance (jp ｖ The method according to claim 8, wherein the numbers are separated into non-zero integer divisors of ).

10. The method according to claim 9, wherein the non-zero integer has at least two divisors.

11. The predetermined distance (jp ｖ The method according to any one of claims 1 to 10, wherein the wavelength of the first seed pulse and the first data pulse is twice or more.

12. The first data pulse (D ０ ) and the second data pulse (D ２ The method according to any one of claims 1 to 11, further comprising modulating the polarization of ).

13. The first seed pulse (S ０ ) and the first data pulse (D ０ The method according to any one of claims 1 to 12, wherein the scanning optical system and the delivery optical system share a common path.

14. The first seed pulse (S ０ ) and the first data pulse (D ０ The process involves generating a third seed pulse and a third data pulse simultaneously with generating a third seed pulse and a third data pulse, wherein the third seed pulse and the third data pulse are spatially separated laser pulses, and the amplitude of the third seed pulse is different from the amplitude of the third data pulse. The first seed pulse (S ０ ) is focused to the first seed location, and the first data pulse (D ０ The method involves focusing the pulses at the first data location and simultaneously focusing the third seed pulse at the third seed location in the substrate, and focusing the third seed pulse at the third seed location in the substrate, wherein the third seed location and the third data location are separated by a predetermined distance along a further scanning path, and the third seed location is located in front of the third data location on the further scanning path. The method according to any one of claims 1 to 13, further comprising:

15. An optical data storage medium (100) obtained by the method described in any one of claims 1 to 14.

16. A transparent substrate (110) and A birefringent voxel (120) embedded in the transparent substrate (110), Equipped with, The optical data storage medium (100) according to claim 15, wherein the birefringent voxel (120) is in the form of an elliptical void having a major axis and a diameter in the range of 1 to 200 nm.

17. An optical data storage medium according to claim 15 or 16, comprising a plurality of layers of birefringent voxels (120) and / or a plurality of birefringent voxels (120) arranged as sighting marks.

18. Pulsed laser source (610), A beam splitter (644) is positioned downstream of the pulsed laser source (610) along the optical path, wherein the beam splitter (644) is Phase difference plate (710), A first polarization-selective diffraction optical system (720) is located downstream of the phase difference plate (710) along the optical path, A second polarization-selective diffraction optical system (730) is located downstream of the first polarization-selective diffraction optical system (720) along the optical path, A beam splitter (644) is provided, A laser writing system comprising, A laser writing system (600) further comprising a focusing optical system (650) located downstream of the beam splitter (644) in the optical path.

19. The laser writing system according to claim 18, wherein the first polarization-selective diffraction optical system (720) includes one of a polarizing grating, a polarizing hologram, or a polarizing lens, and the second polarization-selective diffraction optical system (730) includes one of a polarizing grating, a polarizing hologram, or a polarizing lens.

20. The laser writing system according to claim 18 or 19, wherein at least one of the phase difference plate (710), the first polarization-selective diffraction optical system (720), and the second polarization-selective diffraction optical system (730) is rotatably mounted.

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